Now, the curious fact is that both these equations are equations of (Euclidean) gauge theories. Maxwell's equations have obvious gauge invariance whereas the second equation (REF ) is invariant under the transformation [1]},
\(h_{ab}(x) \rightarrow h_{ab}(x) + \bigg ( \partial _a\partial _b - \frac{1}{4} \delta _{ab} \partial ^2 \bigg ) \phi (x)\) 
